1. Field of the Invention
The present invention relates to a driver and in particular relates to a driver for driving a power switch element.
2. Description of the Related Art
Drivers are necessary for controlling when to turn-off and turn-on power switch elements. For isolation purposes, transformers are used to deliver driving signals to power switch elements such as MOSFET, IGBT etc., as FIG. 1 shows. Transformers only deliver driving energy. Thus, when compared to the size of main circuits, the size of the transformers is relatively smaller. However, as power density and power efficiency requirements increasingly heighten, especially for low frequencies such frequencies lower than 10 kHz, the size and the power consumption of transformers become more of an issue.
The circuit 200 in the FIG. 2 is a conventional driver. The two sides of the transformer T may generate positive pulses and negative pulses according to the rising edges and falling edges of the driving signal 210 by design of the capacitance of the capacitor CY. The positive pulses pass through the diodes D1 and D2 to charge the gate capacitor Ciss of the power switch element Q3 at a high potential. Because the transistor Q4 is closed at this moment, the gate of the power switch element Q3 has no loop to discharge such that a high potential is sustained. The negative pulses pass through the diodes D3 and D4 to turn on the transistor Q4 so that the voltage at the gate of the power switch element Q3 discharge to a low potential through the transistor Q4.
FIG. 3 is a waveform of the circuit shown in FIG. 2. PWMout is the driving signal 210, VCY is a waveform at the capacitor CY in series with the primary winding of the transformer T, Ip is a current waveform of the primary winding of the transformer T, V1/2 and V3/4 are respectively voltage waveforms of the primary winding 270 and second winding 280 of the transformer T, and VG/S is a waveforms at the gate of the power switch element Q3. When the turn ratio of the primary winding and the second winding is 1:1, V1/2 and V3/4 have the same waveforms. The circuit can modulate the driving signal 210 to a signal with a smaller pulse width such as waveforms V1/2 and V3/4. With more narrow pulse signals, the products of the voltages and time of the pulses (i.e. products of the amplitude and width of the pulses) which the transformer T has to process may be smaller. Hence, the size of the transformer T may be reduced in accordance with circuit design.
However, there are a lot of drawbacks in the above mentioned prior art. For example, driving consumption is large. It can be understood that although pulses are delivered by the transformer T at the rising edges and falling edges of the driving signal 210, the primary winding 270 of the transformer T has to keep an average current at almost zero because of the capacitor CY. At the rising edge, the energy stored in the capacitor CY has to be released before a next rising edge in order to sustain operation, which increases power consumption.
For example, the driving signal at the gate of the power switch element Q3 is pulled up slowly. The required waveform at the transformer T is performed through the capacitor CY. Accordingly, the capacitance of the capacitor CY has to match the capacitance of the capacitor Ciss at the gate of the power switch element Q3. A large capacitor CY may not generate pulse signals with a relatively narrow width. Also, a matching capacitor CY may have influential impedance, which blocks energy delivery and reduces rising speed of driving signals. Thus, turn-on power consumption of the power switch element Q3 is increased.
For another drawback, the circuit in the prior art is unreliable. When negative pulses disappear, the gate of the transistor Q4 is at a high-impendence status such that the gate of the power switch element Q3 is at a floating status, wherein an initial status is a low potential rather than an expected low impendence turn-on status. If the power switch element Q3 is charged because of other factor such as the Miller effect etc., the gate of the power switch element Q3 may not sustain a low potential so as to cause malfunction. Although the width of negative pulses may be increased to prolong a low impendence status, a wider pulse width means that the consumption of the transformer must be increased.
Thus, a driver with a small size, high reliability, fast rising speed of driving signals and low driving consumption is called for.